Etoposide is a widely-used antineoplastic agent which inhibits mammalian DNA topoisomerase II isoenzymes. See, F. Drake et al., Biochem. 28:8154 (1989); P. Watt and I. Hickson, Biochem. J. 303:681 (1994); and Y. Pommier, Cancer Chemo. & Pharmac. 32:103 (1993). Various etoposide derivatives have been developed in order to improve antitumor activity, cytotoxicity against drug resistant cells and drug-formulation characteristics including the 4'-O-demethylepipodophyllotoxins bearing C-4.beta.-N-linked substituents. See, Y. Zhang and K. H. Lee, Chin. Pharm. J. 46:319 (1994). The other mammalian DNA topoisomerase, a type I enzyme, is also considered to be a useful therapeutic target. Several selective inhibitors have been identified to date including the antitumor alkaloid, camptothecin, (CPT) and two analogs are currently approved for clinical use in the United States. See, P. Watt and I. Hickson, Biochem. J 303:681 (1994); and A. Chen and L. Liu, Annu. Rev. Pharmacol. Toxicol. 34:191 (1994). However, not all topoisomerase inhibitors of potential clinical value are topoisomerase-type specific. For example, a 7-H-benzopyrido (4,3-b) indole-derivative (inotoplicine), inhibits topoisomerases I and II simultaneously and can circumvent topoisomerase-mediated mechanisms of drug-resistance. See, B. Podderin et al., Mol. Pharmacol. 44:767 (1993).
All of the above-mentioned compounds share a common inhibitory mechanism which is understood in some depth at the biochemical level. Enzyme inhibition involves the trapping of a putative covalent reaction intermediate called a reversible "cleavable complex". The intracellular lesion, presumably a topoisomerase-DNA-drug ternary complex, ultimately leads to cell death. Although the cytotoxic events depend on the particular type of topoisomerase involved, the precise biochemical pathway(s) to cell killing remains to be defined. See, P. Watt and I. Hickson, Biochem. J. 303:681 (1994); A. Chen and L. Liu, Annu. Rev. Pharmacol. Toxicol. 34:191 (1994); and P. Darpa et al., Cancer Res. 50:6919 (1990).